Thirty percent of the Post Sex Survey respondents said that they think threesomes work best when all parties involved are strangers. Photo: Shutterstock

Thirty percent of the Post Sex Survey respondents said that they think threesomes work best when all parties involved are strangers. (Alamy)

In her new, self-published memoir, “Unicorn,” Isabella Martin, 37, a successful corporate lawyer based in an East Coast city, talks about her experience of “The Lifestyle” — the secretive swinging scene in which she sought anonymous sex with strangers. Next month, a new TV reality show, “Secret Sex Lives: Swingers,” covers the same controversial topic. Two-anda- half years after cutting ties from the circuit, Martin (below inset) who wrote her book under a pseudonym and asked The Post not to reveal her real name for professional reasons, tells Jane Ridley how she was drawn into the dark and dangerous world of random hookups, orgies and sex clubs . . .

Breezing into the office on a Monday morning, my co-worker asks what I got up to over the weekend. I reply that I hung out with friends, watched back-to-back episodes of “Mad Men” and walked my dog in the park. “That’s nice,” he says, explaining that he went apple- picking with his family and took in a puppet show at the library.

It sounds so wholesome that I stifle a laugh. I wonder what he would say if I told him how I really spent my weekend. How, for three hours on Saturday night, I had sex with a bunch of strangers in a hotel suite.

At that time, I was part of “The Lifestyle,” in which groups of consenting adults meet for casual sex.

On the swinging scene, my code name was “Unicorn,” since girls like me are considered rare, mythical creatures. Young, attractive and without a husband or steady boyfriend, we make slim pickings.

It started in the fall of 2009, soon after I’d gone through a divorce and slept with about a dozen guys to make up for lost time (my husband was my only sexual partner until I was 33). During that period, I guess I saw sex just like a hot-blooded single guy with no emotional attachments would. I found it difficult to calm my mind and get to sleep if I hadn’t had an orgasm.

I thought about using Craigslist to look for potential partners, but didn’t want to die a horrible death. I tried Match. com, but it was too much effort. I wanted to cut out the bulls- – t and get down to it as quickly as possible. In the end, I paid the $16-amonth basic membership fee for the Web site Adultfriendfinder. com, known as AFF. Some of the members are so terrified of being recognized, they upload profile photographs of themselves with paper grocery bags over their heads!

I researched having sex with a group of guys. There was one man, Joe, who had a “stable” of around 30 swingers, whose replies to my emails were clear and wellwritten. All the other responses were badly spelled and had terrible grammar, like a bad Craigslist ad, which instantly put me off.

I sent him a picture and we ended up planning my so-called “party” for the following Friday evening at a Marriott hotel downtown. There would be two guys as well as Joe and me.

I met Joe in the lobby and we went up to the room. He was an IT consultant in his mid-40s, a person I wouldn’t look at twice in a bar. One of the guys texted to say he was busy at work, so Joe joked that he had to call in “the alternate.” He always had a couple of folks waiting in the wings.

To be honest, the sex that first time was pretty tame, or “vanilla” as I later discovered they call it on the circuit. It wasn’t particularly daring. Afterwards, I wasn’t sure how I felt or whether I’d had a good time. I didn’t feel guilty or dirty, just neutral.

And so it went on. Over the space of 12 months, Joe hosted around four more “parties” in my honor. Sometimes there were two guys, sometimes three or four.

All the time, I knew I was leading a sordid, double life. I confided in a couple of open-minded girlfriends, but nobody else knew. Looking back, I was seriously depressed and mixed-up. I always needed a lot of alcohol to get in the right frame of mind for the parties.

Casual sex seemed like the only way I could escape reality.

The last time I “played” was in November 2010, when I spent a night away with a couple, Bill and Amber, whom I’d had sex with once at a hotel with a bunch of other people. One weekend, they drove me to a place, a split-level ranch in Gettysburg, Pa., which they described as a “sex club.”

That night, I mostly hooked up with girls. There was me and three other women fooling around in the bar while other people watched. A couple of guys were playing pool and, at one point, this jerk walked over to us and shouted: “Hey, can’t you move? You’re blocking my shot!” If he was so focused on the game, what the hell was he doing at a sex club?

The next morning, I woke up feeling hungover, dark and unhappy. There were about six people who’d stayed overnight too, and I walked into the living room to find them watching TV. One woman, Alice, lifted her head. “Here’s our superstar,” she said. “You were so much fun last night. How long have you been in ‘The Lifestyle?’ ”

Somehow her question depressed me. I didn’t want this as my “lifestyle.” The people were nice — they were successful, some of them were attractive, they drove nice cars and had good jobs, but I didn’t fit in. I didn’t want to spend my time dabbling in casual sex with strangers with whom I had no emotional attachment.

Bill and Amber drove me home and I kissed them on the cheek goodbye. I was done. I needed to deal with my emotional problems in a different, healthier and safer way.

The friends I’ve told about that strange episode of my life have stood by me and encouraged me to share my story. I’m gradually starting to date men again — trust me, it’s always just the two of us when we have sex! — and I feel a lot better and more optimistic about myself and my future. I’ve told a few guys about my past and they’ve been curious, but generally accepting of it. They’re mainly just pleased I’ve put it behind me. I’m also seeing a new psychiatrist to work through my issues.

Now I’m living day to day and taking medication to keep my emotions level. Yes, I’d like to settle down with someone, but I’m not sure about ever having children. Who knows where my new life will take me?

“Unicorn,” by Isabella Martin, is available for purchase on Amazon.com.